Direct liquid injection system with on-line cleaning

ABSTRACT

A direct liquid injection system and process which has on-line cleaning of the vaporizers without the need for shutting down the CVD process and thus eliminating down time is provided. The cleaning process includes the steps of providing at least one metalorganic precursor to a first vaporizer to produce a vapor containing the at least one precursor; transporting the vapor to a deposition chamber; periodically interrupting the supply of the at least one metalorganic precursor to the first vaporizer; providing the at least one metalorganic precursor to a second vaporizer to produce a vapor containing the at least one precursor; transporting the vapor to the deposition chamber; and during at least a portion of the time when the supply of the metalorganic precursor is interrupted to the first vaporizer, providing a cleaning fluid (either liquid solvent or cleaning gas plasma) to the first vaporizer, which fluid is effective to at least partially remove deposits of the metalorganic precursor. The process may be either carried out as a batch process, or more preferably, as a continuous process to avoid the need to shut down the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/121,491 filed Jul. 23, 1998.

This application is related to U.S. patent application Ser. No.09/037,235, filed Mar. 10, 1998, the disclosure of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to a direct liquid injection system whichprovides for on-line cleaning, and more particularly to a system andprocess for the controlled deposition of metal oxide layers usingchemical vapor deposition techniques.

Semiconductor devices such as dynamic random access memories (DRAMs)have undergone substantial decreases in size and increases in chargestorage density over the past several years, and it is expected thatthese trends will continue into the future. In order to increasecapacity while decreasing size, DRAM designs have become increasinglycomplex. One problem has been the design of capacitors in the DRAM whichwill hold the necessary electrical charge representing stored data.

Oxides of silicon have conventionally been used as the dielectricmaterials in such DRAM capacitors. Silicon oxides, however, haverelatively low dielectric constants and limited charge storagedensities. Accordingly, there has been an effort in the art to identifymaterials having higher dielectric constants which are suitable for usein DRAM designs. Interest in ferroelectric materials such as bariumstrontium titanates (Ba_(1−x)Sr_(x)TiO₃, known as BSTs) and leadzirconate titanates (Pb(Zr_(x)Ti_(1−x))O₃, known as PZTs) has grownbecause such materials have relatively higher dielectric constants thansilicon oxides, are structurally stable, and can be prepared using knowntechniques.

Of the many chemical and physical deposition techniques used in the artto form thin film layers of such ferroelectric materials, metal organicchemical vapor deposition (MOCVD) using direct liquid injection appearsto hold the most promise. For example, Desu et al, U.S. Pat. Nos.5,431,958 and 5,527,567 teach MOCVD techniques, including direct liquidinjection ('567 patent), to provided layered ferroelectric films for themanufacture of capacitors. Si et al, U.S. Pat. No. 5,629,229, also teachthe manufacture of DRAMs using MOCVD techniques.

In MOCVD, the metalorganic precursors which are used are dissolved inliquid solvents which are then pumped in precise proportions to avaporizer. The vaporized precursors are then sent to a CVD chamber wherethey are deposited on a substrate. The composition and properties of thedeposited films of the ferroelectric materials are highly dependent onthe ability of the direct liquid injection system to supply the correctproportions of precursors to the vaporizer and thence to the CVDchamber. Other variables in the system will also affect the compositionand properties of the deposited films and include the condition of thevaporizer, the temperature of the vaporizing surfaces in the vaporizer,the concentrations of the metalorganic precursors in the solvent, andlocal temperature variations on the substrate surface in the reactor.The local temperature variations as well as accumulation of precursorcompounds on the surfaces of the vaporizer both affect vaporizationefficiency and can cause fluctuations in the composition and propertiesof the films which are deposited.

Such accumulations of precursor compounds and oxidative reactionproducts of the precursor compounds have the tendency to build-up overtime and clog both the outlet to the vaporizer as well as the directliquid injection mechanism. This leads not only to undesirablevariations in the ratios of the precursor compounds which are deposited,but also to possible clogging and shutdown of the deposition process. Toaddress these problems, Gardiner et al, U.S. Pat. No. 5,362,328, teachthe use of a cleaning subsystem in a chemical vapor deposition processin which a solvent is supplied to the vaporizer to solubilize anydeposited compounds and flush them away. However, the Gardiner et alcleaning subsystem requires that the CVD process be periodically shutdown during the cleaning cycle. Accordingly, the need still exists inthis art for a direct liquid injection system which provides for on-linecleaning without the need for shutting down the CVD process.

SUMMARY OF THE INVENTION

The present invention meets that need by providing a direct liquidinjection system which has on-line cleaning of the vaporizers withoutthe need for shutting down the CVD process, and thus eliminating downtime. The present invention also provides alternative sources ofcleaning fluid which may be selected to remove metalorganic precursorand oxidation product residues which are deposited in the vaporizers.

In accordance with one aspect of the present invention, a process forthe on-line cleaning of a direct liquid injection system is provided andincludes the steps of providing at least one metalorganic precursor to afirst vaporizer to produce a vapor containing the at least oneprecursor; transporting the vapor to a deposition chamber; periodicallyinterrupting the supply of the at least one metalorganic precursor tothe first vaporizer; providing the at least one metalorganic precursorto a second vaporizer to produce a vapor containing the at least oneprecursor; transporting the vapor to the deposition chamber; and duringat least a portion of the time when the supply of the metalorganicprecursor is interrupted to the first vaporizer, providing a cleaningfluid to the first vaporizer, which fluid is effective to at leastpartially remove deposits of the metalorganic precursor and oxidationproducts. Preferably, the cleaning fluid is effective to removesubstantially all of the deposits and residue of the at least onemetalorganic precursor and any oxidation products which may have formed.

The process may be either carried out as a batch process, or morepreferably, as a continuous process. Thus, when the supply ofmetalorganic precursor is interrupted to the first vaporizer forcleaning, the supply to the second vaporizer is initiated so that thereis a continuous flow of vaporized precursor being supplied to thedeposition chamber. That supply of metalorganic precursor is maintainedto the second vaporizer until a buildup of deposits or residue isdetected. Then the procedure is reversed by resuming the supply of theat least one metalorganic precursor to the first vaporizer andinterrupting the supply of the at least one metalorganic precursor tothe second vaporizer. During at least a portion of the time when thesupply of the metalorganic precursor is interrupted to the secondvaporizer, a cleaning fluid is provided to the second vaporizer, whichfluid is effective to at least partially remove deposits of themetalorganic precursor and oxidation products, and preferably, iseffective to substantially completely remove deposits and residues ofthe metalorganic precursor and any oxidation products which may haveformed.

In a preferred form, the at least one metalorganic precursor isdissolved in a liquid solvent carrier which is supplied to thevaporizers. The cleaning fluid may comprise a liquid solvent for the atleast one metalorganic precursor. The cleaning fluid may be recoveredand recycled after it has been passed through the vaporizer.Alternatively, the cleaning fluid may comprise a gas plasma which is anetchant for the deposits of metalorganic precursor and oxidationproducts. The gas plasma may be formed in a conventional manner, suchas, for example, using microwave energy to form the plasma. The gasplasma is formed from an etchant gas which is preferably selected fromthe group consisting of NF₃, CIF₃, and HF. The plasma is formed at atemperature and at a pressure which permits it to be sufficientlylong-lived to effect its cleaning function in the vaporizers.

Additionally, the vaporizer may be cleaned by sequentially supplyingdifferent cleaning fluids to it. Thus, the vaporizer may be cleaned byfirst using a gas plasma which is then followed by a solvent-containingfluid. Alternatively, the vaporizer may be cleaned by first using asolvent-containing cleaning fluid which is followed by a gas plasmatreatment.

The step of monitoring the build up of deposits in the first vaporizerand interrupting the supply of the metalorganic precursor is preferablydesigned to operate when such buildup reaches a predetermined level. Thedegree of buildup of metalorganic precursor deposits may be measured bymonitoring the flow rate of the vapor from the first vaporizer. Whensuch flow rate drops below a predetermined level, the supply ofmetalorganic precursor is interrupted, and cleaning fluid is sent to thevaporizer.

The present invention also provides an apparatus for a direct liquidinjection system with on-line cleaning which includes a source of atleast one metalorganic precursor; first and second vaporizers for the atleast one metalorganic precursor; a chemical vapor deposition chamberfor receiving vaporized metalorganic precursor; a source of cleaningfluid for removing deposits of the at least one metalorganic precursorfrom the first and second vaporizers; and a controller for directing theat least one metalorganic precursor to either the first or secondvaporizer, for periodically interrupting the flow of the at least onemetalorganic precursor to the first or second vaporizer, and forinitiating a flow of the cleaning fluid to the vaporizer which has hadits supply of metalorganic precursor interrupted. The system alsopreferably includes monitors in or adjacent to the first and secondvaporizers for monitoring the build up of deposits of the at least onemetalorganic precursor. The monitors may comprise flow rate measurementdevices.

In a preferred form, a valve is positioned between the source of the atleast one metalorganic precursor and the first vaporizer, and anothervalve is positioned between the source of the at least one metalorganicprecursor and the second vaporizer. The controller regulates theoperation of the valves by feedback control from the monitors.

The present invention has the advantage of being able to operatecontinuously while providing cleaning fluids which are suitable andeffective to remove even the most difficult to remove deposits andresidues of the metalorganic precursors and any oxidation products whichmay have formed. These and other features and advantages of theinvention will become apparent from the following detailed description,the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

The single drawing figure depicts, in schematic form, a preferredconfiguration for the direct liquid injection system of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a system and process for using directliquid injection to deposit single or multicomponent oxide films onsemiconductor devices. Examples of multicomponent oxides include, butare not limited to, barium strontium titanate (BST), bismuth strontiumtantalum oxide (SBT), lead zirconate titanate (PZT), and lanthanum leadzirconate titanate (PLZT). These ferroelectric materials exhibit highdielectric constants and low current leakage, are chemically andphysically stable, and can be deposited in a dense film at highdeposition rates using conventional techniques to provide good stepcoverage of substrates. As such, these materials find use in capacitorstructures in semiconductor devices such as random access memories aswell as in other devices using ferroelectric materials includingpyroelectric detectors, ultrasonic sensors, and electro-optic devicesincluding optical switches and optical displays. As used herein, theterm “substrate” means any material with sufficient load-bearingcapability and internal strength to withstand the application of a layeror layers of additional materials during a fabrication process. As usedherein, the term encompasses silicon structures such as silicon wafersand semiconductor devices, including semiconductor devices in theprocess of fabrication. The term “silicon wafer” means either the lowestlayer of semiconductor material in a wafer or a wafer having additionallayers or structures formed thereon.

The metalorganic precursors used in the practice of the presentinvention are preferably alkyls, alkoxides, β-diketonates, ormetallocenes of the corresponding metal elements which make up themulticomponent metal oxide films. Suitable metalorganic precursors foruse in a MOCVD process include those compounds which exhibit high vaporpressure at low vaporization temperatures, low decompositiontemperatures, a large “window” between vaporization and decompositiontemperatures, no contamination from organic constituents of theprecursors, stability under ambient conditions, and nontoxicity. Theseproperties of the precursors may be modified and adjusted to some extentby the choice of the particular organic substituents. For example,volatility of a metal β-iketonate can be varied by varying the alkylgroup to which it is attached.

As shown in the drawing Figure, the system 10 includes a source 12 of atleast one metalorganic precursor. While a single source is shown forpurposes of illustration, it will be appreciated that the number ofprecursors may vary depending upon the particular metal oxide film whichis to be formed. For example, there may be three or more separatesources of different metalorganic precursors. In such a case, asdescribed in commonly-assigned copending U.S. patent application Ser.No. 09/037,235, filed Mar. 10, 1998, the disclosure of which is herebyincorporated by reference, the individual metalorganic precursors arepreferably supplied in liquid form by dissolving them in suitable liquidcarriers. The separate streams of liquid precursors are then mixed priorto being supplied to the vaporizer. Suitable solvent carriers formetalorganic compounds include tetrahydrofuran (C₄H₈O), isopropanol(C₃H₇OH), tetraglyme (C₁₀H₂₂O₅), and mixtures thereof. Typically, theconcentration of metalorganic precursors in the solvent carrier will bebetween about 0.01 to about 1.0 mole per liter of solvent.

The metalorganic precursor, or blend of metalorganic precursors, is sentto first vaporizer 14. Vaporizer 14 is preferably one which can quicklyheat the precursor (preferably to at least 250° C.) and efficientlycause the precursor and liquid carrier to flash vaporize. One suitablevaporizer is commercially available from MKS Instruments of Andover, Md.and utilizes heated metal disks. Vaporizer 14 may also include a port(not shown) for mixing an inert gas such as nitrogen with the vaporizedprecursor to carry it into the chemical vapor deposition chamber. Theinert gas which is supplied to vaporizer 14 may also be used to vary thecomposition, and thus the vaporizing characteristics, of the precursor.

After vaporization, the metalorganic precursor is sent to chemical vapordeposition (CVD) chamber 16. Chamber 16 may be either a hot wall or coldwall MOCVD reactor of the type conventionally used in this art. Asubstrate (not shown) is placed into chamber 16 which is heated to atemperature of between about 300° to 800° C. and maintained at apressure of between about 1.0×10⁻⁴ to 100 torr. In chamber 16, thevaporized metalorganic precursor(s) decompose and are deposited assingle or multi-component metal oxides on the surface of the substrate.Typically, multi-component oxide films are deposited to a thickness offrom between about 10 to about 1000 Å, and more preferably from about 50to about 300 Å. Such deposition takes from about 30 seconds to about 20minutes, and more typically from about 1 to 2 minutes.

While the system is designed to supply precise quantities ofmetalorganic precursors to vaporizer 14, and thence to chamber 16,because of the conditions in vaporizer 14, there is a tendency for themetalorganic precursor to deposit and solidify on the surfaces of thevaporizer. Further, some oxidation products of the precursor may haveformed as well. Accumulation of precursor and oxidation compounds on thesurfaces of the vaporizer, including the injector and vapor outlet, bothaffect vaporization efficiency and can cause fluctuations in thecomposition and properties of the films which are deposited.

To insure that the vaporized precursor stream continuously supplies thecorrect amount of vaporized precursor to chamber 16 for deposition and,for multicomponent embodiments, maintains its desired stoichiometricratio of components, the vaporized stream is continuously (orintermittently) monitored by monitoring device 18 which, in theillustrated embodiment, is located downstream from first vaporizer 14.It will be appreciated, however, that depending on the particular typeof monitoring device, the device may be located in or adjacent to thevaporizer. Monitoring device 18 may monitor any of a number of physicalor process parameters of the vaporized precursor stream. A suitablemonitoring device is a flow rate monitor for measuring the volumetricflow rate of the precursor. Monitor 18 supplies information to acontroller 22. Controller 22 may be either a programmable logiccontroller or a programmable general purpose computer.

When that flow rate is measured to have dropped below a predeterminedvalue, the drop in flow indicates a buildup of deposits and/or residueson the surfaces of the vaporizer. In the prior art, such a buildupnecessitated the shutdown of the system for periodic cleaning of thevaporizer surfaces. However, the system of the present invention ispreferably designed to operate continuously. For that purpose, a secondvaporizer 20 is provided. When monitor 18 supplies information tocontroller 22 which indicates such a buildup, controller 22 acts tointerrupt the flow of metalorganic precursor to first vaporizer 14 byclosing valve 24. Simultaneously, controller 22 opens valve 26 toredirect the supply of metalorganic precursor(s) to second vaporizer 20.

Controller 22 may also at this time, or a later point in time, begin thecleaning cycle for first vaporizer 14 to remove the buildup of depositsand/or residues. A cleaning fluid in the form of either a gas or aliquid may be utilized. For example, a liquid solvent for themetalorganic precursor(s) and oxidation products may be stored inreservoir 28 and its flow to first vaporizer 14 may be initiated byopening valves 30 and 32, respectively. A series of pumps (not shown)insures the proper flow of all components through the system. Thesolvents may be the same solvents described above as carriers for themetalorganic precursor(s). Spent solvent, along with the metalorganicprecursor deposits and residues which have been removed from vaporizer14, is taken from first vaporizer 14 through line 34 and condenser 36 tospent solvent reservoir 38. The spent solvent may either be disposed ofor sent to a recovery station 40 where impurities and contaminants areremoved from the solvent, and the solvent is recycled to solventreservoir 28 for reuse.

Alternatively (or in addition to the use of liquid solvent), thecleaning fluid may be a gas plasma using a cleaning gas which is anetchant for the metalorganic precursor deposits and residues in thevaporizer. It has been found that a cleaning gas plasma is effective toreact with and remove deposits which are difficult to remove completelyusing liquid solvents. The gas plasma is generated in generator 42 usinga cleaning gas from source 44. Controller 22 opens valve 46 to initiatethe flow of plasma to vaporizer 14. Thus, the present inventioncontemplates that the vaporizer may be cleaned using a liquid solvent, agas plasma, or a sequence in which both cleaning fluid are utilized inseries.

The gas plasma may be formed in a conventional manner, such as, forexample, using microwave energy to form the plasma. The gas plasma isformed from an etchant gas which is preferably selected from the groupconsisting of NF₃, CIF₃, and HF. The plasma is formed at a temperatureand at a pressure which permits it to be sufficiently long-lived toeffect its cleaning function in the vaporizers. The spent gas plasma andremoved deposits of metalorganic precursor(s) are sent through line 34for disposal or recovery.

Once the deposits and residues are cleaned from vaporizer 14, controller22 shuts off the flow of cleaning fluid. The flow of vaporizedmetalorganic precursor from second vaporizer 20 is monitored bymonitoring device 48, located downstream from second vaporizer 20, whichreports information to controller 22. Once a buildup of deposits insecond vaporizer 20 is detected, controller 22 resumes the supply ofmetalorganic precursor to first vaporizer 14, and interrupts the flow ofmetalorganic precursor to second vaporizer 20. Then, the cleaning cyclefor second vaporizer 20 is initiated by beginning the flow of eitherliquid solvent from reservoir 28 by opening valve 50, or, alternatively,gas plasma is introduced into second vaporizer 20 by opening valve 52.The cleaning cycle for second vaporizer is as was previously describedwith respect to first vaporizer 14.

Thus, the system and process of the present invention may be operated ina batch process. However, most preferably, the process and system areoperated to provide a continuous supply of vaporized metalorganicprecursor species to CVD chamber 16. This is accomplished by providingon-line cleaning of vaporizer chambers while maintaining the flow ofmetalorganic precursor to the CVD chamber.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. An apparatus for a direct liquid injection systemwith on-line cleaning comprising: a source of at least one metalorganicprecursor; first and second vaporizers for said at least onemetalorganic precursor; a chemical deposition chamber for receivingvaporized metalorganic precursor; a source of cleaning fluid forremoving deposits of said at least one metalorganic precursor from saidfirst and second vaporizers; and A controller for directing said atleast one metalorganic precursor to either said first or secondvaporizer, for periodically interrupting the flow of said at least onemetalorganic precursor to said first or second vaporizer, forredirecting said at least one metalorganic precursor to the other ofsaid first or second vaporizers, and for initiating a flow of saidcleaning fluid to the vaporizer which has had its supply of metalorganicprecursor interrupted.
 2. An apparatus as claimed in claim 1 in whichsaid source of cleaning fluid comprises a liquid solvent.
 3. Anapparatus as claimed in claim 1 in which said source of cleaning fluidcomprises a gas plasma.
 4. An apparatus as claimed in claim 3 includingmonitors in or adjacent to said first and second vaporizers formonitoring the build up of deposits of said at least one metalorganicprecursor.
 5. An apparatus as claimed in claim 4 in which said monitorscomprise flow rate measurement devices.
 6. An apparatus as claimed inclaim 4 in which said monitors communicate with said controller.
 7. Anapparatus as claimed in claim 1 including a valve positioned betweensaid source of said at lest one metalorganic precursor and said firstvaporizer and a valve positioned between said source of said at leastone metalorganic precursor and said second vaporizer.
 8. An apparatus asclaimed in claim 7 in which said controller regulates the operation ofsaid valves.